Title: Photoreceptor dysfunction associated with rhodopsin mislocalization Abstract: Rhodopsin mislocalization is observed in various blinding disorders including syndromic and non-syndromic retinitis pigmentosa. In most of these disorders rhodopsin mislocalizes to the inner segment (IS) plasma membrane (PM). Growing evidence suggests that PM mislocalization of rhodopsin is the root cause of photoreceptor degeneration, but how such mislocalization causes rod photoreceptor degeneration remains unknown. In this project, we will test the hypothesis that mislocalized rhodopsin disrupts PM homeostasis thereby causing dysfunction and degeneration of rod photoreceptor neurons. In rod photoreceptors, rhodopsin is synthesized at an extremely high rate and delivered to the base of the outer segments (OSs). This high rate of synthesis is balanced with a high rate of catabolism. Rhodopsin-containing disk membranes are shed at the tip of the OSs, engulfed, and digested by the retinal pigment epithelial (RPE) cells. When rhodopsin mislocalizes, a massive amount of rhodopsin is delivered to the PM of the ISs, where rhodopsin-containing membranes have no apparent contact with RPE cells. Nevertheless, we recently found that mislocalized rhodopsin is actively eliminated from the PM while new rhodopsin molecules are continuously delivered to this structure. The mechanism of elimination will be the subject of this study (Aim 1). Photoreceptor cells are terminally differentiated neurons that survive during the entire lifespan of vertebrates, including humans. Therefore, the contents of photoreceptor cells must be continuously renewed. How this renewal occurs for the OS structure is well-established, but has not been studied for the IS. We will address the renewal mechanism of IS PM proteins in rod photoreceptors and investigate the pathological process of disrupting the IS PM protein homeostasis by massive mistrafficking of rhodopsin there (Aim 2).